Wire bonding is a process in which semiconductor chips are wired after they are mounted onto a substrate, typically a leadframe, to electrically connect the semiconductor chips and the substrate. The wire bonder has a capillary that is clamped to the tip of an ultrasonic transducer, which serves to secure bonding wire to a connection point on the semiconductor chip and to a corresponding connection point on the leadframe. To form the wire connection between the connection points, an end of the bonding wire protruding out of the capillary is first melted into a ball. Thereafter, the melted ball is secured to the connection point on the semiconductor chip by means of pressure and ultrasonic energy in a process called ball bonding. The wire is then pulled through to the required length, formed into a wire loop and welded to the connection point on the leadframe in a process called wedge bonding. After securing the wire to the connection point on the leadframe, the wire is torn off and the bonding cycle is repeated.
FIG. 1(a) is a side view of a conventional wire bonding system 10 wherein a bonding stage 12 is located close to a wire bonding area. The bonding stage 12 supports an optics system 14 that is used for viewing the bonding area, and a camera 16 that is configured to receive images transmitted by the optics system 14.
The bonding stage 12 also holds a transducer 18 that has a capillary 19 clamped to its tip for performing wire bonding. A device 20 to be wire bonded at the bonding area using the capillary 19 is supported on a heater block 22 that is operative to bring the device 20 to a suitable wire bonding temperature. Heat 24 is generated by the heater block 22 and such heat 24 dissipates to the surrounding ambient air.
The heat 24 may be absorbed by the bonding stage 12, optics system 14 and the transducer 18 due to their proximity to the heater block 22. Thus, the heat 24 may cause the bonding stage 12, optics system 14 and transducer 18 to expand.
FIG. 1(b) is a side view of the conventional wire bonding system 10 wherein the bonding stage 12 is located further away from the wire bonding area because another location in the bonding area is being bonded. In this case, since the heater block 22 may no longer be directly underneath the components that were previously heated, the bonding stage 12, optics system 14 and transducer 18 may start to cool down and contract.
Therefore, it is observed that when the bonding stage 12 is moved close to the heater block 22, the temperature of the bonding stage 12 will be raised by the heat transferred from the heater block 22. Conversely, when the bonding stage 12 moves further away from the heater block 22, its temperature will be relatively lower. As a result, the temperature of the whole bonding stage 12 will be continually changed during bonding. The temperature change of the bonding stage will create unwanted deviations in the position of both the optics system 14 and the transducer 18 by thermal expansion and contraction. This leads to inaccuracy of bonding on the semiconductor chip as well as on the leadframe. As far as possible, the temperature of the bonding stage should desirably be kept constant during bonding.
A traditional approach to temperature control is to couple a heater to the bonding stage. The heater may be used to raise the temperature of the bonding stage 12 by heating it, and to lower the temperature of the bonding stage 12, the bonding stage 12 may be air-cooled naturally with ambient air by deactivating the heater. A problem with this approach is that the rate of heating and cooling are different since only heating is actively conducted. Cooling will generally take a longer time. Another problem is that heating and cooling is localized at the position of the heater, which builds up a thermal gradient that leads to thermal stress. It would be preferable to be able to both heat and cool the whole of the bonding stage dynamically.